Add concurrent tests for graphdatabases
Add SynchronizedGraphDatabase wrapper to help with using thread-unsafe backends
Merge pull request #525 from dwins/issue-520-tinkergraph-concurrency

Issue 520 tinkergraph concurrency
Added SQLite storage module.

* Base module for all SQLite backends
* Backend implementation based on the Xerial SQLite driver
* Implementations for object, graph, config, and staging storage
set sqlite as the default backend